P.S. To create a E2B USB drive, you should use the \MAKE_E2B_USB_DRIVE.cmd shortcut (but in your case you will probably get the same error because RMPrepUSB just calls RMPartUSB which is the same program used by the batch file anyway.

RMPartUSB actually uses the Windows Format API but it needs to be passed a drive letter (e.g 'told to format drive L:).

So, RMPartUSB wipes the drive, creates the partitions and then asks Windows what drive letter the new partition is. Then it calls the Windows Format_NTFS API with the drive letter. It looks like Windows is not assigning a drive letter to the raw partition.

Instead of using RMPrepUSB\RMPartUSB to format the drive, you can just use the Windows Format utility - it does the same thing!